Method of preparing an impregnated cathode with an enhanced thermionic emission from a porous billet and cathode so prepared

ABSTRACT

A method is provided of preparing an impregnated cathode with enhanced  thionic emission from a porous billet by impregnating the billet with a suitable impregnant in the presence of an oxygen deficient compound.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

FIELD OF INVENTION

This invention relates in general to a method of preparing an impregnated cathode with enhanced thermionic emission from a porous billet and to a cathode so prepared and in particular to such a method wherein the impregnation is made in the presence of an oxygen deficient compound.

BACKGROUND OF THE INVENTION

Heretofore it has been known that electron emission could be obtained from a porous billet as for example a porous tungsten billet that had been impregnated with a barium containing compound such as Ba₃ Al₂ O₆. The Ba₃ Al₂ O₆ impregnant reacts with the wall of the porous tungsten billet generating free barium. The free barium then migrates to the surface by Knudsen flow to give electron emission.

The difficulty with this concept is that there is no mechanism for the generation of electrons. The concept of barium migrating to the surface to give off electrons is too generalized in that if 15-20 mg of barium containing compound gave free barium that was responsible for electron emission, the cathode would cease to give emission within minutes.

SUMMARY OF THE INVENTION

The general object of this invention is to provide a method of making a cathode having a more enhanced emission.

It has now been found that the foregoing object can be attained by impregnating a porous billet in the presence of an oxygen deficient compound. Such a compound that is similar in structure to superconductor deficient oxides would then generate electrons.

The oxygen deficient compound can be considered as a compound in which a site is available for an oxygen atom but the site is not occupied by an oxygen atom. When the oxygen site is unoccupied, the valence of the remaining metals drops to a lower valence state.

The oxygen deficient compounds used in the invention include SCWO₄, AlWO₄, MoO₂, WO₂ and mixed oxides of rhenium and iridium.

In the method of the invention, regeneration of the impregnant must occur for the cathodes to have a long life of 80,000 to 200,000 hours. Then too, oxygen deficient compounds must either be present in the cathode or must be generated in the cathode. The oxygen deficient compounds that are generated or present react once they have acquired negative charge by the method used above, with Ba and/or BaO to form oxygen sufficient compounds with the release of electrons that are responsible for electron emission. Additives such as Ir, Os, and Rh react in such a way as to increase emission by reacting to generate oxygen deficient compounds such as WO₂. Moreover, intermediate oxygen sufficient products formed in the chemical reactions can be used as impregnants providing they generate oxygen deficient compounds.

A method of regeneration of the impregnant is illustrated using Ba₂ Al₂ O₆ to form an intermediate that reacts to form WO₂ and releases free Ba leaving an unstable intermediate (oxygen deficient) Ba₂ Al₂ O₄. This compound in the presence of 2W reacts to form 2 Al +2 WO₂ +2 Ba. When these compounds that are generated are added to the above free Ba and WO₂ the total becomes 2 Al +3 WO₃ +3 Ba and the overall equation becomes

    Ba.sub.3 Al.sub.2 O.sub.6 +3 W→3 Ba+2 Al+3 WO.sub.2

The 3 Ba+2 Al +3 WO₂ reacts with each other to form Ba₃ Al₂ O₆ +3 W that are the original starting compounds. The equations for this are illustrated below

    Ba.sub.3 Al.sub.2 O.sub.6 +W→Ba+WO.sub.2 +Ba.sub.2 Al.sub.2 O.sub.4

    Ba.sub.2 Al.sub.2 O.sub.4 +2 W→2 Ba+2 Al+2 WO.sub.2

Combining the two equations above gives

    Ba.sub.3 Al.sub.2 O.sub.6 +3 W→3 Ba+2 Al+3 WO.sub.2

The compounds that were formed from a series of steps convert back to the impregnant and W again.

This regeneration scheme is ideally illustrated. The formation of WO₂ (not illustrated) in the above scheme occurs when a WO₂ attacks the impregnant Ba₃ Al₂ O₆ to remove one oxygen to form WO₃ and a Ba₃ Al₂ O₅ Molecule that is oxygen deficient. The WO₃ can react with the Al generated to give Al₂ (WO₄)₃ which in the presence of W gives AlWO₄ and WO₂.

The reactions are shown by chemical equations 3Ba₃ Al₂ O₆ +W→3Ba₃ Al₂ O₅ +WO₃ The Ba₃ Al₂ O₅ generated is oxygen deficient; two electrons are now present where the sixth oxygen was present in the Ba₃ Al₂ O₆ structure.

The WO₃ formed f rom above can react with the Al generated previously to give WO₂ and AlWO_(4;4) WO₃ +2Al →2 WO₂ +AlWO₄. The WO₂ and AlWO₄ are both oxygen deficient compounds.

The impregnants used for the porous billet must be of the type A_(x) B_(y) O_(z) where A is a very electro positive metal (more active than B). B is a metal that converts over to its most stable oxide in the presence of tungsten (W) or other active billet material such as molybdenum. The 0 is oxygen in the above formula. The subscript Z must be such that the valence of A times its subscript is equal to subscript of the oxygen (z) divided by the absolute value of the valence of oxygen (2). The value of the subscript on the oxygen (z) can be one less than this amount if one of the oxygen's are replaced with a pair of electrons. An example would be Ba₃ Al₂ O₆ and Ba₃ Al₂ O₅ (1 pair of electrons is substituted for the oxygen that is attached to the aluminum).

The A which is more active than B attacks the B oxide and converts it to a pure metal and the A in turn converts to its stable oxide.

The active B in the presence of W03 reacts to form two oxygen deficient compounds B_(y) WO₄ (where Y =+1) and WO₂ in a W billet. When the Ba and BaO generated previously react with the oxygen deficient materials to form oxygen sufficient materials such as BaWO₄ along with Al f or example, the materials generated are recycled into the regeneration process to continue the process of electron emission.

Various other methods of generation of oxygen deficient compounds in cathodes have been demonstrated.

Reaction of an oxygen sufficient tungstate or molybdate of B (such as Al or Sc) with W. An example is

    2 W+Al.sub.2 (WO.sub.4).sub.3 →2AlWO.sub.4 +WO.sub.2

Another illustration of formation of an oxygen deficient compound is through the reaction of B oxides, B metal and WO₃ such as Al₂ O₃ +5Al+9WO₃ →7AlWO₄ +WO₂ +W

Another illustration is the B stable oxide (Al2O3 for example) with WO₃ and WO₂ as shown Al₂ o₃ +WO₂ +WO₃ →2AlWO₄

Another illustration is Al₂ (WO₄)₃ +Al→3AlWO₄

A_(x) B_(y) O_(z) compounds must be able to form the oxygen deficient compounds and then convert to oxygen sufficient compounds which are capable of joining the regeneration cycle.

Since products such as oxygen deficient compounds such as WO₂, SCWO₄, MoO₂ are formed for example as well as other intermediate products such as free Al, free Sc, oxides such as SC₂ O₃, Al₂ O₃ and WO₃ that help in the formation of oxygen deficient compounds, they can be added in molar ratios such that the combination with Ba and BaO will contribute to low temperatures operation and fast warm-ups for cathodes.

Application of pulverized pieces of alloys such as low melting Al₅ Ba₄ in molar ratio suitable for maximum emission with materials listed above gives maximum emission.

A W or W-Al alloy can be used for the porous billet. W-Ir, W-Os etc can also be used as the porous billet.

In lieu of an impregnated porous billet by itself, one may employ a top layering emission.

A top layering emission includes two separate electron generators; the impregnanted billet itself, and the top layered material. The current density is a sum of both generators.

Both Ba and BaO that are generated in the billet below the top layered billet migrate to the layered top to form intermediates and oxygen deficient compounds similar to those produced in the porous billet. The Ba and BaO that usually escapes from the billet is now used by the top layered portion of this billet.

To initiate top-layering reactions, formation of compounds such as SC₂ (WO₄)₃, or their presence initially in or on a portion of the top layer must be present. Also present must be W such that SC₂ (WO₄)3 +W→2SCWO₄ +2 WO₂. Both products are oxygen deficient and in the presence of Ba and BaO react to form oxygen sufficient compounds and electrons.

Scandium metal, for example, that can be generated when Ba reacts with SC₂ (WO₄)₃ can participate in the reaction by reacting with SC₂ (WO₄)₃ to form SCWO₄, an oxygen deficient compound.

Oxygen deficient compounds such as SCWO₄ and WO₂ must be present initially or must be formed for emission to occur. Some preparation of top-layering could include mixtures of [Sc₂ O₃ /WO₃ /W], [Sc₂ (WO₄)₃ /W], [Sc₂ (WO₄)₃ /ScWO₄ /W/WO₃ ]for example. Only mixtures that give oxygen deficient compounds can be considered for top-layering.

Both Ba and BaO must enter the top layering to obtain maximum emission. AlWO₄,for example needs Ba, WO₂ needs BaO for maximum emission generating electrons.

When oxygen deficient WO₂ reacts with 2 BaO, Ba is generated. This makes for better emission because the Ba is generated within the top layer and does not have to be generated within the porous billet. Possibility of a Bao generator at the bottom of an enriched WO₂ layer to give high emission can be made.

Al and WO₃ mixtures have been demonstrated to give oxygen deficient compounds AlWO₄ and WO₂. Mixtures of Al and WO₂ can be used in top-layering in the presence of tungsten.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Since the emission of the impregnated cathodes involve the formation of oxygen deficient compounds, a method to maximize emission can be obtained by (1) Adding the oxygen deficient compounds to the impregnant; (2) Adding compounds such as Al₂ (WO₄)₃ or Sc₂ (WO₄)₃ which in the presence of W react to form WO₂ and AlWO₄ or SCWO₄ which are oxygen deficient compounds or (3) Adding composites of 1 and 2 above. Examples are illustrated below.

EXAMPLE 1

The example below illustrates the use of intermediate compounds that are formed on the surface and interior of the cathode during operation. Use of intermediates such as WO₃, Al₂ O₃ and alloys such as Al₁₃ Ba₇ to initiate the chemical reaction at temperatures lower than that when only the impregnant such as Ba₃ Al₂ O₆ is present.

Ba₃ Al₂ O₆, WO₃, Al₂ O₃, and Al₁₃ Ba₇ alloy are mixed in such a way that the molar combinations are 2 mole Ba₃ Al₂ O₆, mole WO₃, 1 mole Al₂ O₃ and 0.05 to 0.20 moles of Al₁₃ Ba₇. This mixture is crushed and then ball milled for two hours. Twenty to forty milligrams of the above molar mixture is mixed with 200 to 300 mgs of tungsten powder. The mixture is ball milled and placed into an isostatic compressor with 60,000 lb/in² into a billet. Xray and Auger Spectroscopy tests are run on the billet to determine the distribution of the powder mixture throughout the billet. Sintering the billet at 700° C. for 10 minutes in hydrogen, vacuum or inert gas such as argon prepares the billet for a cathode environment.

EXAMPLE 2

Another example illustrated below uses the standard impregnant Ba₃ Al₂ O₆ with oxygen deficient compounds such as WO₂ and AlWO₄.

Ba₃ Al₂ O₆, WO₂, AlWO₄ and an alloy of aluminum and barium such as Al₁₃ Ba₇ are mixed in such a way that the molar combination is 2 moles Ba₃ Al₂ O₆, 1 mole WO₂, 1 mole AlWO₄ and 0.05 to 0.2 mole Al₁₃ Ba₇. The mixture is ball milled for two hours and then a mixture of 200 to 300 mg of tungsten powder is mixed with 20 to 40 mg of the above molar combination of Ba₃ Al₂ O₆, WO₂, AlWO₄ and Al₁₃ Ba₇. The mixture is isostatically compacted into a billet, and Xray and Auger Spectroscopy test are done to determine the distribution of the powders through the billet. Sintering at 700° C. in H₂, vacuum, or an inert gas such as argon for 10 minutes prepares the billet for a cathode environment.

EXAMPLE 3

Other mixtures for impregnation would include mixtures of Ba₃ Al₂ O₆ and Al₂ (WO₄)₃ in molar concentrations of 1 mole Ba₃ Al₂ O₆ and 1 mole of Al₂ (WO₄)₃ with 0.05 to 0.1 mole Al₁₃ Ba₇.

Sintering, mixing, and compacting of the above powder with W powder are similar to EXAMPLES I and 2.

EXAMPLE 4

The use of intermediates with barium scandates, and scandium intermediates can also be used as in a cathode impregnant.

Illustrations Are:

a. Ba₂ SC₂ O₅ with WO₃, SC₂ O₃ such that the molar concentration is 2 moles Ba₂ Sc₂ O₅ with 1 mole WO₃ and 1 mole of SC₂ O₃.

b. The Ba₆ Sc₆ O₁₅ /WO₃ and SC₂ O₃ such that the molar concentration is 2 moles Ba₆ SC₆ O₁₅, 2 moles WO₃, and 0.1 to 0.3 mole of Sc₂ O₃.

c. The Ba₃ Sc₄ O₉ with WO₃ and Sc₂ O₃ such that the molar concentration is 2 moles Ba₃ Sc₄ O₉, 1 mole WO₃ and 0.1 to 0.2 mole of Sc₂ O₃.

Sintering mixing and compacting of the above powder with W powder are similar to examples 1 and 2 above.

EXAMPLE 5

The use of oxygen deficient compound such that WO₂ and ScWO₄ with the barium scandates illustration of example 4 is as follows:

1. Ba₂ Sc₂ O₅ with WO₂ and ScWO₄ such that the molar combinations are 1 mole Ba₂ Sc₂ O₅, mole WO₂ and 1 mole ScWO₄.

2. Ba₆ Sc₆ O₁₅ with WO₂ and SCWO₄ such that the molar concentration is 2 moles Ba₆ Sc₆ O₁₅, moles WO₂ and 0.1 to 0.3 mole of ScWO₄.

3. Ba₃ Sc₄ O₉ with WO₂ and SCWO₄ such that the molar combination is 1 mole Ba₃ Sc₄ O₉, mole WO₂ and 0. 1 to 0. 3 mole SCWO₄.

Sintering, mixing and compacting the above powders with W powder are similar to Examples 1 and 2.

EXAMPLE 6

This example involves all the mixtures found in Examples 1 through 5 but adding the mixtures to a tungsten cup of known volume and geometric size. Instead of isostatically compacting the mixtures, the mixtures can be solidified by CVD reactions of W from W(CO)₆ and the melting of aluminum. The intermediate is 0.05 mole Al₂ (WO₄)₃, 0.5 mole of Al₁₃ B₇ and 1 mole of W with 1 mole of Ba₃ Al₂ O₆.

We wish it to be understood that we do not desire to be limited to the exact details Of construction shown and described for obvious modifications will occur to a person skilled in the art. 

What is claimed is:
 1. A cathode having an enhanced thermionic emission including a porous billet and a sintered mixture of an impregnant with other compounds within such porous billet that react to form an oxygen deficient compound at least as one of their products, wherein the impregnnt is Ba₃ Al₂ O₆ and wherein the other compounds within the porous billet that react with the Ba₃ Al₂ O₆ impregnant are Wo₃, Al₂ O₃ and Al₁₃ Ba₇ alloy.
 2. A cathode having an enhanced thermionic emission including a porous billet and a sintered mixture of an impregnant with other compounds within such porous billet that react to form an oxygen deficient compound at least as one of their products, wherein the compounds are mixed within the porous billet in the mole ratio of 2 moles Ba₃ Al₂ O₆ to 1 mole WO₃ to 1 mole Al₂ O₃ to 0.05 to 0.20 mole of Al₁₃ Ba₇ alloy.
 3. A cathode having an enhanced thermionic emission including a porous billet and a sintered mixture of an impregnant and at least one oxygen deficient compound within the porous billet wherein the impregnant is Ba₃ Al₂ O₆ and the impregnant is mixed with the oxygen deficient compounds WO₂ and AlWO₄ and the alloy Al₁₃ Ba₇.
 4. A cathode having an enhanced thermionic emission including a porous billet and a sintered mixture of an impregnant and at least one oxygen deficient compound within the porous billet wherein the compounds are mixed in the ratio of 2 moles Ba₃ Al₂ O₆ to 1 mole WO₂ to 1 mole AlWO₄ to 0.05 to 0.2 moles of Al₁₃ Ba₇ alloy. 